This paper presents a market-based, multi-robot planning capability, designed as part of a distributed, layered architecture for multi-robot control and coordination. More specifically, we are developing an extension to the traditional three-layered robot architecture that enables robots to interact directly at each layer --- at the behavioral level, the robots create distributed control loops; at the executive level, they synchronize task execution; at the planning level, they use market-based techniques to assign tasks and allocate resources. The market-based planning layer of each robot has two main components: (1) a trader that participates in the market, auctioning and bidding on tasks; (2) a scheduler that determines task feasibility and cost for the trader, and interacts with the executive layer for task execution. This paper focuses on the planning level, detailing the architecture, our current implementation, and planned future extensions. We show how the architecture (in particular, the planning layer) has been applied to a Mars exploration scenario involving the characterization of scientifically "interesting" rocks. We also present preliminary simulation results exploring market and scenario parameters.